1. Field of the Invention
This invention relates to a method of manufacturing of base material particles with a porous surface by joining a number of core materials (particles), which can be used, for example, as an ebullition heat transfer surface.
2. Description of the Related Art
A method of manufacturing an ebullition heat transfer surface will be explained as an example of base material particles with a porous surface.
As is well known, the quantity of heat Q (Kcal/h) propagated from a heat transfer surface to the adjoining liquid is calculated by the following formula: EQU Q=.alpha..multidot..lambda..multidot..DELTA.
where a is the transfer rate (Kcal/m.sup.2 h .degree.C.) based on ebullition, .lambda. is the surface area (m.sup.2) of the heat transfer surface, and .DELTA. is the difference (.degree.C.) between surface temperature Tw (.degree.C.) of the heat transfer surface and the temperature T(.degree.C.) of the liquid.
A heat transfer surface with good heat transfer characteristics is defined as a surface where a small temperature difference (.DELTA.T) causes a large heat transfer from the transfer surface to another material (for example the liquid). In other words, a heat transfer surface with good heat transfer characteristics can be defined as one with a large a.multidot..lambda. value in the above formula. A conventional method adopted to increase the heat transfer area .lambda. is to attach fins to the heat transfer surface, or to roughen the heat transfer surface with a sandblast.
In order to increase the heat transfer rate, a porous surface is used based on the following assumption. The assumption is that, with an ebullition phenomena, the governing factor of the heat transfer is the movement of liquid and steam surrounding the heat transfer area, especially the effect of disturbances and latent heat of liquid caused by steam bubbles leaving the heat conducting area which makes the ebullition heat transfer ratio a far greater than the effect of the convective heat transfer discussed above without generation of steam bubbles (without phase change). For example, the forced convection heat transfer ratio of air is approximately 100 (Kcal/m.sup.2 h .degree.C.), where the ebullition heat transfer ratio of water is around 10,000 (Kcal/m.sup.2 h .degree.C.). Because the steam bubbles originate from the liquid adjoining the heat transfer surface, fresh liquid must be supplied to the heat transfer surface after the generation of the steam bubbles, preventing a sudden decrease of the heat transfer ratio a due to film ebullition, where dried steam covers the heat transfer surface.
Therefore, in order to increase the ebullition heat transfer ratio a the number of bubble generation points on the heat transfer surface must be increased and liquid must be constantly supplied to the ebullition surface. On a porous surface, the steam in the cavities forms a bubble core and the cavities are connected inside the porous layer, which supplies fresh liquid to the bubble generation points and increases the heat transfer ratio a.
FIG. 12 is a cross-sectional view of a conventional heat transfer surface with porous surface, which has been manufactured in the following manner. Metal particles (1) that consist of sintered metal and bond (such as phenol aldehyde resin) are mixed and coated on the smooth heat transfer body (2); the above particles are layered in a porous manner and heated to a high temperature; the metal particles are sintered on the heat transfer body (2); and heated again to reduce and remove the bond. In this way the porous layer (31) is made to form a porous surface (41) in which a large number of cavities (5) may contain steam.
FIG. 13 is an example of a cross-sectional view of a porous heat transfer area shown in Japanese Patent Publication sho 61-610. It has an appropriate number (one in the case of FIG. 13) of core materials (particles) (7) (for example metal such as copper or nickel, inorganic material such as glass, or polymers like styrene) on the surface of the heat transfer body (2). The core material is dipped in, for example, copper plating liquid, which forms a metal coat (8). With the metal coat, the above-mentioned core material (particle) (7) is retained on the heat transfer body (2), which forms the porous layer (31) forming the porous surface (41).
The conventional porous surface (41) formed with the sintered metal (1) described in FIG. 12 is suitable for use as an ebullition heat transfer surface. Process control of homogeneous mass-production of the porous surface, however, becomes complex because control of the atmosphere and bonding material during sintering is difficult, and the shape is complex since the metal particles are melting each other.
The other conventional method described in FIG. 13 also has problems. The joining strength among the core material (particles) and between the heat transfer body and between individual core material (particles) is weak, which causes separation of the core material (particles) (7) due to thermal expansion or bending of the heat transfer body. Moreover, large scale plating instrumentation is necessary to dip the heat transfer body for mass-production.